Crystal and method of fabricating same



Nov, 29 "21949 (G. A. ARGABRITE ETAL, 224399393 CRYSTAL AND METHOD OF FABRICATING SAME Filed June 2, 1944 INVENTORS George A. Argobrite T. Finley Burke BY WW ATTORNEY Patented Nov. 29, 1949 CRYSTAL AND METHOD OF FABRICATING SAME George A. Argabrite and T. Finley Burke, San Diego, Calif asslgnors to the United States of America as represented by the Secretary of the Navy Application June 2, 1944, Serial No. 538,434

Claims. 1 This invention relates to large size crystals a method of forming the same.

The industry has found many uses for large size crystals which, because of the usually comand plex method of growing, are diflicult to produce.

As the necessary size of the crystals increases,

the percentage of usable and flawless structures produced becomes less. These difliculties have been found to be particularly important in the production of piezoelectric crystals used in radio and sound transmission and reception. Since the operating frequencies of the crystals used in such work is a direct function of size, it is obvious that for low-frequency operation, large size crystals are necessary. Even if grown by the usual methods in temperature-controlled baths, which is not at all feasible for some types of crystals in any case, the process is still both expensive and time consuming.-

To overcome these disadvantages, many types of glues, cements, etc. have been suggested for physically joining one or more smaller size crys-'- tals. Thus, for example, two crystals, each resonant at kilocycles, are joined end to end, to produce a crystal twice as large, with a resonant frequency of approximately 5 kilocycles. However these attempts have generally been unsuccessful because of the failure of the glued or cemented joint in one or more respects. In some, it was found that when the crystal was excited,

the joint failed mechanically because of the physical vibration to which, it was subjected. In others, the properties of the joint were such as to introduce large losses which necessarily impaired the eiiiciency of operation.

To overcome these objections, the present invention consists of a joined crystal in which the crystal substance is used as a cement or glue. It has been found that the similarity in chemical composition and physical properties of the crystals and the joining section produces a, new size crystal with characteristics, both mechanical and electrical, much like that of a pre-grown crystal of the same size. It not only appears that the lattice structure suffers only slight discontinuity at the joint but also that the joint is such that no new substantial losses are introduced.

The method by which these large size crystals is formed is exceedingly simple and consists in merely heating the faces to be joined until the crystals begin to melt and then forcing said adjacent faces together under a slight pressure until the molten substance has cooled.

In the drawings the figure shows one form of device partialy cut away which may be used to form the large size crystals.

The invention may be practiced on a large variety of crystalline substances, but is particularly useful for forming piezoelectric crystals, including Rochelle salt (KNaC4H4Os-4HZO), beryllium sulphate, beryllium oxide, primary ammonium phosphate, primary sodium phosphate, primary potassium phosphate, isomorphous mixtures of phosphate and certain arsenates, and in the science of spectroscopy.

The technique used in performing the invention is simple; but although its limits are rather critical, they are diflicut of definition. The faces of the two crystals to be joined must first be cleaned with a solvent not harmful to the crystal in order that foreign bodies or particles will not affect the subsequent cooling or recrystallization. Heat is then applied to the faces for a length of time and at a temperature just sufllcient to cause melting of a thin layer of the crystal on the face to be joined. Care must be taken that the melting is confined, insofar as possible, to the surface area and that such melting is uniform over this area. Preferably, the melted portion of the crystals should not be heated more than enough to give it a sticky consistency so that it will not drip or run off. The temperature used and the length of heating time will necessarily depend upon the melting temperature of the crystalline substance. It should also be noted that the whole body of the crystal should preferably be raised to as high a temperature as possible without I melting or otherwise damaging it, in order that the subsequent cooling does not occur rapidly.

When the two surfaces to be joined are melted as described above, they should be brought together very quickly and held together under a slight pressure, as might be exerted with the fingers. This pressure should be maintained until the molten substance has completely cooled, and in some cases, for a considerable period afterwards. It has also been found advantageous to excite the crystal (or crystals) mechanically (or electrically, in the case of piezoelectric crystals) during the cooling.

To illustrate the process, but without limiting the invention, the joining of two piezoelectric, Rochelle salt, crystals will be described. Since this substance melts at from approximately C., the surfaces, after having been cleaned with benzine or alcohol, are brought up to the melting temperature. When the surfaces become sticky, they are quickly brought together and held under pressure (a few ounces per square inch is sufficient) in a smallvise. For best results they should be maintained thusly for approximately 3-10 minutes. The addition of heat to the Rochelle salt drives of! the water of crystallization and decomposes the Rochelle salt in accordance with the following formula:

Thus, the resulting salts apparently form a super-saturated solution in the free water. lJpon cooling, the two salts again combine with some of the water and the following is obtained (from the above equation):

Any excess free water probably evaporates during or after cooling. Thus, chemically, the substance of the joint is very similar to that of the original crystals, and the new large crystal behaves very like one large continuous unit.

The assembly of the figure shows one simple method of forming the crystal Joint. A small tray is formed of any convenient material such as Bakelite with a bottom I and twoside sections 2, held together by screws or bolts 3. This tray is formed just wide enough to receive two sliding laws 4, which are the exact width of the crystals to be Joined. Each of the laws is made flexible and with a groove to receive one of the crystals 5. Thumb screws 8 are mounted in the jaws and are adapted to tighten the jaws to hold the crystals firmly in place. The sides 2 of the tray are slotted at the mid point to receive a. slider 1 against which the adjacent faces of the crystals to be joined are. held. Small springs are mounted onthebottom l ofthetrayandarearrangedto act against the Jaws and thus hold the crystal against the slider I.

In operation, the crystals I are clamped in the Jaws I, positioned in the tray, and are thus forced together by springs l. The slider I, made of metal aother suitable material, is heated to the desired temperature. It is then forced down between the crystals and left for a short time until the faces of the crystals begin to melt. As it is withdrawn, the springs 8 force the two crystals snugly together where they are held until the joint cools.

The arangement shown in the figure is illustrative only and any convenient method may be used. The one illustrated has proved successful, however, and its simplicity insures quick and dependable operation.

Having described our invention, we claim:

1. A method of joining two crystals each having a face conforming in shape to a. face on the other comprising the steps of melting a thin surface layer on both of said faces; pressing said faces together under slight pressure; and maintaining said pressure until said surface layers cool to room temperature.

2. A method of joining two crystals each having a face conforming in shape to a face on the other comprising the steps of melting a thin surface layer on both of said faces; pressing said faces together under slight pressure; agitating said crystals" while maintaining such pressure; and maintaining such pressure until said surface layers cool to room temperature.

3. A crystal comprising two crystal elements each having a bonding surface conforming to a similar surface on the other; and a thin, fused, homogeneous bonding layer between said elements coextensive with at least one of said surfaces; said layer consisting of a melted and recooled portion of the body of at least one of said elements.

4. A method of joining two crystals each having a face conforming in shape to a face on the other, comprising the steps of applying heat to the surface area of at least one of said faces to bring about a fusion of a thin layer thereon; lightly pressing said faces together; and maintaining said I pressure until said surfaces cool to room temperature.

5. A method of joining two crystals each having REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 843,515 Bier Feb. 5, 1907 1,737,019 Nicolson Nov. 26, 1929 2,168,943 Pfundt Aug. 8, 1939 2,222,906 Hentzen Nov. 26, 1940 2,306,054 Guyer Dec. 22, 1942 

